LONDON (Reuters) - Activist investor Edward Bramson is pushing to shutter the majority of trading activities undertaken by Barclays' investment banking unit, in a radical plan to cut costs and boost returns at the British lender, three sources told Reuters. Investors briefed by the New York-based financier, who took a surprise 5 percent stake in Barclays in February , said Bramson was planning to axe all parts of its investment banking operation that did not directly serve corporate clients...